The present invention relates to vehicle service equipment such as is commonly utilized in vehicle repair shops, and more particularly, to vehicle service equipment such as wheel balancers and tire changing systems which have been configured to provide quantified measures of vehicle wheel assembly lateral forces.
Even when a vehicle wheel assembly, consisting of a wheel rim and a tire mounted thereon, is properly balanced, non-uniformity in the construction of the tire, as well as runout in the wheel rim itself, can cause significant vibration forces as the vehicle wheel assembly rolls over a road surface under a vehicle load. In order to reject tires having significant non-uniformity in their construction, many tire manufacturers inspect their tires on complex tire uniformity machines, such as disclosed in U.S. Pat. No. 6,116,084 to Fischer et al. The measurement procedures implemented in tire uniformity machines typically requires a load roller to be linearly pressed against the surface of a rotating tire at a precisely controlled angle (i.e. perpendicular to the tire surface). To ensure repeatable and consistent measurements, this linear actuation and controlled contact angle must be maintained across various sizes and shapes of tires, and requires a complex mechanical setup.
If a tire is found to be non-uniform in some manner, the rolling characteristics may be improved by a process in which the tire tread surface is selectively ground away. However, it is not uncommon for tires to produce vibration forces (not related to imbalance) of 20 pounds or more as they roll on a smooth road even after such a grinding procedure. Grinding procedures can also be perceived negatively by vehicle owners as they reduce the remaining tread and hence useable life of the tire.
Some vehicle wheel assemblies develop forces in the lateral (or axial) direction when rolling straight ahead on a flat road surface. This condition may cause a vehicle to steer away from a straight line. Sources of lateral forces include tire conicity, ply steer, and the combination of the two, known as total lateral force. Tire conicity may be envisioned by considering a vehicle wheel assembly to assume the shape of a truncated cone as it rolls. Based on geometry, such a configuration will always generate a force towards the apex of the cone regardless of the direction in which the vehicle wheel rotates. Thus, tire conicity is a force component which does not change direction with reverse rotation when measuring tire lateral residual forces. By definition:
  Conicity  =                    TLF        CW            +              TLF        CCW              2  
where TLFCW is the total lateral force of the vehicle wheel assembly measured in the clockwise direction, and TLFCCW is the total lateral force of the vehicle wheel assembly measured in the counter-clockwise direction. Tire conicity is believed to be caused by the placement of internal tire components, such as belts off-center about the circumference of the tire, resulting in the tire having one sidewall which is stiffer than the other.
Tire ply steer lateral forces result from the influence of the plies in a tire in generating forces which can steer a vehicle from a straight line course. These forces are theorized to be related to the direction of the cords in the outermost ply, and hence, as the direction of rotation is reversed, the direction of the force generated by the outermost ply also changes. Thus tire ply steer is a force component which changes direction with reverse rotation when measuring tire lateral residual forces. By definition:
  Plysteer  =                    TLF        CW            -              TLF                                                  ⁢          CCW                      2  
where TLFCW is the total lateral force of the vehicle wheel assembly measured in the clockwise direction, and TLFCCW is the total lateral force of the vehicle wheel assembly measured in the counter-clockwise direction.
Other lateral forces which may be measured include the peak-to-peak lateral force variation, the lateral first harmonic force variation, as well as other higher lateral harmonic force variations.
The variations in radial and lateral forces during the rotation of a vehicle wheel assembly are usually caused by differences in the stiffness and/or geometry of the vehicle wheel assembly about its circumference or tread centerline. If these differences are slight, the radial and lateral force variations, and therefore the degree of tire conicity, will be insignificant and their effects unnoticeable when the vehicle wheel assembly is installed on a vehicle. However, when these differences reach a certain level, the radial and/or lateral force variations may be significant enough to cause rough riding conditions and/or difficult handling situations. Furthermore, an excessive tire conicity value will cause a rolling tire to pull to one side.
Conditions such as tire conicity and ply steer cannot be corrected during the balancing of a vehicle wheel assembly by the attachment of balancing weights. Temporary corrections to such conditions can be made by altering the shape of the tire through the use of grinding machines and the removal of tire tread material from specifically identified regions on the tire. Examples of such systems are shown in U.S. Pat. No. 3,948,004 to Gruber, U.S. Pat. No. 4,112,630 to Brown, Jr. and U.S. Pat. No. 5,645,465 to Vannan, III. However, such operations do not result in a long-term solution to the effects of lateral forces, and may result in shortened tire life as the remaining tire tread wears.
Information identifying or quantifying the measurement of lateral force present in a rotating vehicle wheel assembly is useful to automotive service personnel, consumers, and tire manufacturers. Such information may be utilized to diagnose vehicle handling problems, to recommend placement positions for a vehicle wheel assembly on a vehicle, and to facilitate quality control and/or tire warranty services.
Accordingly, it would be advantageous to provide a vehicle service system, such as a tire changer or a wheel balancer, with a measurement method which is capable of providing a quantifiable measure of the lateral forces present in a rotating vehicle wheel assembly, without the need for a complex mechanical measurements system employing precisely aligned and linearly actuated load rollers.